Fix flaky test where decoding could throw IllegalStateException
Fix flaky test where decoding could throw IllegalStateException

for example when we call flush at the of the stream for x-COMPOUND-TEXT charset on
jdk8 and the input stream ends in special control character (see
https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/CTEXT/ctext.html#Control_Characters)

This change relaxes the assertion on decoding result to make sure that
we only assert string content if the byte array is welformed and
accepted by nio's decoder

Fixes #4561

rossabaker/http4s,http4s/http4s,rossabaker/http4s